A radioimmunoassay system for the glycoprotein beta 1SP1, one of the pregnancy-associated proteins, has been set up. beta 1SP1 levels in normal pregnancy have been established transversally. Clearance (t1/2) from the circulation postpartum ranged from 1.4 to 1.8 days. Several patients with trophoblastic disease, e.g., mola hydatidosa, choriocarcinoma and embryonic cell carcinoma, were followed longitudinally. Human chorionic gonadotropin (hCG), a highly reliable tumor marker in trophoblastic disease, has been determined concomitantly by radioimmunoassay. In all instances, hCG proved to be a better indicator of the presence of a tumor, because by the time beta 1SP1 became undetectable, hCG could still be detected. Samples from a large multicenter study on trophoblastic disease were studied: out of 100 positive for hCG, beta 1SP1 could be detected in 86; however, out of 21 samples in which hCG was not detectable, 1 was found to be positive for beta 1SP1. In a number of samples from patients with bronchial carcinoma, no beta 1SP1 could be detected.